1. Field of the Invention
This invention relates to rare-earth anisotropic powders and magnets consisting essentially of Fe-R-B alloys (R is at least one of neodymium and praseodymium or at least one of them and one or more other rare-earth elements) and their manufacturing processes.
2. Description of the Prior Art
Recently developed rare-earth-and-iron-based anisotropic magnets having excellent magnetic properties can be divided into the following three categories according to their manufacturing processes:
(1) A sintered anisotropic magnet made by forming, sintering and heat-treating a powder prepared by grinding a cast alloy to a fineness of the order of single crystals of approximately 3 .mu.m and oriented in a magnetic field (Japanese Provisional Patent Publication No. 46008 of 1984). PA1 (2) A bonded isotropic magnet made by forming a mixture of an isotropic powder, which is prepared by grinding flaky thin ribbons, approximately 20 to 30 .mu.m in thickness, obtained by a melt quenching process, and a resin (Japanese Provisional Patent Publication No. 64739 of 1984); a bulked isotropic magnet made by hot-pressing an isotropic powder into a mass of high density and a bulked anisotropic magnet made by hot-upsetting the high-density bulked isotropic magnet (Japanese Provisional Patent Publication No. 100402 of 1985, IEEE Trans. Mag. Vol. MAG 21, No. 5 1985 (1985)); and a bonded anisotropic magnet made by forming in a magnetic field a mixture of an anisotropic powder, which is prepared by grinding the bulked anisotropic magnet, and a resin (Japanese Provisional Patent Publication No. 7504 of 1989). PA1 (3) A bulked anisotropic magnet made by plastically deforming a cast ingot by hot upsetting or other processes (Japanese Provisional Patent Publications Nos. 203302 of 1987 and 704 of 1989).
Made of a powder ground to a fineness of the order of single crystals, the sintered anisotropic magnet (1) has highly-aligned magnetic domains, producing as great a magnetic strength as 35 to 45 MGOe in terms of maximum energy product. But its thermal stability is low because its crystal grain size is as large as about 10 .mu.m and its coercive force depends on nucleation (i.e., the coercive force is determined when new reverse-domain walls appear from grain boundaries etc.). When the sintered anisotropic magnet is ground to a powder, the coercive force drops significantly under the influence of the oxidization and strain at the surface of the powder (Y. Nozawa et al. J. Appl. Phys. Vol. 64 No. 10 5285-5289 (1988)). Several methods heretofore proposed to suppress the post-grinding drop in the coercive force by changing the sintering conditions and applying heat treatment to the ground powder (C. R. Paik et al. IEEE Trans. Mag. Mag-23 No. 5 2512 (1987)), and other measures have not succeeded in solving problems of low magnetic properties, thermal stability and corrosion resistance.
The anisotropic magnet (3) too does not have good thermal stability because its crystal grain size and mechanism to provide coercive force are similar to those of the sintered anisotropic magnet (T. Shimoda et al. Proceeding of the Tenth International Workshop in Rare-Earth Magnets and Their Application, (1), 389 (1989)). This process is unsuitable for the making of anisotropic powders because grinding lowers magnetic properties.
In contrast, the anisotropic powder and magnet (2) maintain their magnetic properties even after grinding because their crystal grain size is fine and their coercive force depends on pinning (i.e., the coercive force is determined when domain walls at grain boundaries etc. move to other places getting out of position). As a result of the plastic deformation applied for the attainment of anisotropy, however, their crystal grains are flattened. Because the plastic deformation takes place at high temperatures, in addition, crystal grains grow larger to reduce absolute coercive force, while increasing its temperature coefficient to -0.60%/.degree.C. As a consequence, the irreversible loss of magnetic flux becomes as great as about -30% after heat-treated at 140.degree. C. (when permeance coefficient=-2) and the magnet becomes no longer suited for practical use. Here the irreversible loss of magnetic flux means the fraction by which the magnetic flux of a specimen magnetized at room temperature, heated to a given temperature and kept at that temperature for a given time, decreases when it is cooled to room, temperature.
A technology to improve thermal stability by adding gallium, Ga, to R-Fe-(Co)-B alloys was disclosed (Japanese Provisional Patent Publication No. 7504 of 1989). But the addition of gallium improves thermal stability by increasing intrinsic coercive force to between 19 and 21 kOe. Magnetizability decreases with increasing coercive force. Being much more expensive than neodymium, Nd, etc., in addition, gallium raises the total material cost. Thus, gallium is not practically preferable additive.
The manufacturing processes of anisotropic magnets disclosed in Japanese Provisional Patent Publication Nos. 100402 of 1985 and 7504 of 1989 grind flaky thin ribbons, ranging between approximately 20 and 30 .mu.m in thickness, obtained by a melt quenching process. The obtained powder is compacted by hot pressing and then formed into bulked anisotropic magnets by hot-upsetting. These processes are complicated. Because final shapes are difficult to obtain by upsetting, in addition, formed pieces must be cut or ground and polished into the desired shape. The process to grind an upset anisotropic magnet into an anisotropic powder too is complicated and unsuited for mass production. To eliminate the shortcomings of these processes, the inventor et al. invented a simple process for manufacturing anisotropic powders that is suited for mass production (Japanese Patent Application No. 256550 of 1988).
Japanese Provisional Patent Publication No. 39702 of 1989 discloses a process for making anisotropic magnets by subjecting powders of R-Fe-B-Cu-M alloys (M is at least one element chosen from the group of zirconium, niobium, molybdenum, hafnium, tantalum and tungsten) obtained by a melt quenching process to hot plastic working. With the content of R limited to 12 atomic percent or under, the process improves plastic workability by taking advantage of the effect of copper. Because the presence of zirconium or niobium is indispensable, however, plastic deformation and anisotropy are difficult to occur if R is kept more than 12 atomic percent.
As is obvious from the above, the conventional rare-earth-iron-based anisotropic magnets involve many problems. Because of the poor thermal stability, for example, they are unsuited for such applications as motors used at high temperatures. Even is their thermal stability is improved by addition of gallium, their magnetzability is impaired through an increase in their intrinsic coercive force. Besides, expensive gallium raises the total material cost. And their manufacturing processes are complicated.